The field of biometrics has rapidly evolved in recent years as the need for fast, accurate personal identification has increased. Motivated primarily by national security concerns, law enforcement agencies have expanded well beyond traditional fingerprint identification to, for example, classifiers based on facial topology, vocal patterns, iris structure, and retinal vessel landmarks.
A very good biometric identifier may be one that is measured quickly and easily and is not easily susceptible to cosmetic modification or falsification. This coupled with the work of Daugman and Downing (Proc. R. Soc. Lond. B 268, 1737-1740 (2001)) has led to the development of multiple commercially available iris recognition systems.
Using the iris in recognition systems is currently becoming more commonplace. However, iris recognition is a complete paradigm shift from traditional fingerprint recognition systems and, therefore, can be costly to implement and to reconstruct data bases of existing identification files. Furthermore, it may be possible to thwart iris identification using, for example, a patterned contact lens.
U.S. Pat. No. 5,751,835 to Topping et al. discusses the use of the tissue of the fingernail bed as a unique identifier. The tissue under a fingernail is composed of characteristic ridges that form during gestation and are unique to each individual. While the ridges may increase in size as the individual grows, the spacing between the ridges remains consistent over time.
Optical coherence tomography (OCT) is a noninvasive imaging technique that provides microscopic sectioning of biological samples. By measuring singly backscattered light as a function of depth, OCT fills a valuable niche in imaging of tissue structure, providing subsurface imaging with high spatial resolution (˜2.0-10.0 μm) in three dimensions and high sensitivity (>110 dB) in vivo with no contact needed between the probe and the tissue.
In biological and biomedical imaging applications, OCT allows for micrometer-scale, non-invasive imaging in transparent, translucent, and/or highly-scattering biological tissues. The depth ranging capability of OCT is generally based on low-coherence interferometry, in which light from a broadband source is split between illuminating the sample of interest and a reference path. The interference pattern of light reflected or backscattered from the sample and light from the reference delay contains information about the location and scattering amplitude of the scatterers in the sample.
In time-domain OCT (TDOCT), this information is typically extracted by scanning the reference path delay and detecting the resulting interferogram pattern as a function of that delay. The envelope of the interferogram pattern thus detected represents a map of the reflectivity of the sample versus depth, generally called an A-scan, with depth resolution given by the coherence length of the source. In OCT systems, multiple A-scans are typically acquired while the sample beam is scanned laterally across the tissue surface, building up a two-dimensional map of reflectivity versus depth and lateral extent typically called a B-scan. The lateral resolution of the B-scan is approximated by the confocal resolving power of the sample arm optical system, which is usually given by the size of the focused optical spot in the tissue. The time-domain approach used in conventional OCT has been successful in supporting biological and medical applications, and numerous in vivo human clinical trials of OCT reported to date have utilized this approach.
An alternate approach to data collection in OCT has been shown to have significant advantages both in reduced system complexity and in increased signal-to-noise ratio (SNR). This approach involves acquiring the interferometric signal generated by mixing sample light with reference light at a fixed group delay in the wavelength or frequency domain and processing the Fourier transform of this spectral interferogram from a wavenumber to a spatial domain. Two distinct methods have been developed which use this Fourier domain OCT (FDOCT) approach. The first, generally termed Spectral-domain or spectrometer-based OCT (SDOCT), uses a broadband light source and achieves spectral discrimination with a dispersive spectrometer in the detector arm. The second, generally termed swept-source OCT (SSOCT) or optical frequency-domain imaging (OFDI), time-encodes wavenumber by rapidly tuning a narrowband source through a broad optical bandwidth. Both of these techniques may allow for a dramatic improvement in SNR of up to 15.0-20.0 dB over time-domain OCT, because they detect all of the backscattered power from the entire relevant sample depth in each measurement interval. This is in contrast to previous-generation time-domain OCT, where destructive interference is typically used to isolate the interferometric signal from only one depth at a time as the reference delay is scanned.